Method for characterizing a read head based on write-induced magnetic read center shifts and read sensitivity changes in the read head

ABSTRACT

A method for characterizing a head in a hard drive assembly based on a magnetic read center shift and a read sensitivity change of the head. A disk attached to a hard disk drive assembly having a plurality of tracks each having a track centerline is provided. One of the tracks has a plurality of sectors, one of the sectors having a servo field with an A burst, a B burst and a C burst, where the A burst and the C burst have a common boundary located at a track centerline and where B burst has a center that is positioned along the track centerline. Each sector also has a data field. The read element is aligned with the B servo burst and a write excitation field is applied to the write element. The amplitudes of an A burst and a C burst located in a following sector of the track are then read to provide an A burst amplitude value and a C burst amplitude value. The steps of writing and reading are repeated for a predetermined number of times to obtain a set of A burst amplitude values and a set of C burst amplitude values. A value representing an instability of the read element based on the set of A burst amplitude values and the set of C burst amplitude values is then calculated. The head is characterized based on the value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to disk storage systems andmore particularly, to a method for characterizing a read head based onthe correlation of write-induced magnetic read center shifts and readsensitivity changes in the read head.

2. Description of the Related Art

Disk drives are magnetic recording devices used for the storage ofinformation. The information is recorded on concentric tracks on eithersurface of one or more magnetic recording disks. The disks are rotatablymounted to a spin motor and information is accessed by means ofread/write heads that are mounted to actuator arms which are rotated bya voice coil motor. The voice coil motor is excited with a current torotate the actuator and move the heads. The read/write heads must beaccurately aligned with the storage tracks on the disk to ensure properreading and writing of information.

Dual element transducers are being increasingly utilized in hard diskdrives because they have greater aerial densities than single elementtransducers. Dual element transducers include a single inductive writeelement and a separate read element which is constructed from amagneto-resistive material. Such dual element transducers are commonlyreferred to as magneto-resistive (MR) heads.

The magnetic material in the read or write element of an MR head isgenerally characterized by a relative permeability μ, where μμ₀ =B/H, μ₀being the magnetic constant (4π×10⁻⁷), B being the magnetic field and Hbeing the magnetic field intensity. However, magnetic materials arecomposed of individual domains with local magnetizations equal to thesaturation magnetization of the material. When a head is used forwriting or reading, the rotation of magnetization within these domains,or the shift of domain walls, constitutes the head response to magneticfields.

Where there are multiple domain configurations in the head, the domainconfiguration in an MR read element can be reset from one state to otherstates by a write field excitation provided by a write element of the MRhead. Such changes in the domain configuration of an MR read elementoften results in changes in readback characteristics of the head, forexample, the amplitude, pulse width and shape, as well as the amplitudeasymmetry of the readback signal. It has also been observed that theread center for an unstable MR head shifted frequently and randomly fromits original position to different positions after write operations.Similarly, when a gated write excitation is applied to an unstableinductive head, its magnetic read center also shifts. Such magnetic readcenter shifts can cause track misregistration in disk drives. Theproblem is compounded in disk drives with high storage density. It isthus important to accurately characterize and determine the instabilityof read heads.

One current technique of characterizing instability of read heads is theuse of a coefficient of variance of the track average amplitude (TAA),COV(TAA), which is expressed as follows:

    COV(TAA)=σ(TAA)/Avg(TAA)×100%

where σ(TAA) is the standard deviation of the TAA and Avg(TAA) is theaverage value of the TAA, where each TAA is measured after a writeexcitation.

However, several factors attributing to the instability are notconsidered in the above approach. Changes in readback amplitudes for theMR head are due to the combined effects of the read sensitivity changeand the magnetic read center shift of the head. FIG. 1 illustrates thestandard deviation of the magnetic read center shift under gated writeexcitations vs. COV(TAA) for a group of MR heads. The average write andread widths for these heads were 3.3 μm and 2.5 μm, respectively. Asshown in FIG. 1, the correlation between COV(TAA) and the standarddeviation of the read center shifts, σ(shifts), is quite poor. Thus, theuse of COV(TAA) to determine the instability of read heads, as well asits use to determine write-induced track misregistration for disk drivesis inaccurate and can be misleading.

Accordingly, there is a need in the technology for a simple and accuratemethod for characterizing a read head based on write-induced magneticread center shifts and read sensitivity changes in the read head, whichwill facilitate reduction of track misregistration, as well as theclassification and qualification of read heads.

BRIEF SUMMARY OF THE INVENTION

A method for characterizing a head in a hard drive assembly based on amagnetic read center shift and a read sensitivity change of the head. Adisk attached to a hard disk drive assembly having a plurality of trackseach having a track centerline is provided. One of the tracks has aplurality of sectors, one of the sectors having a servo field with an Aburst, a B burst and a C burst, where the A burst and the C burst have acommon boundary located at a track centerline and where B burst has acenter that is positioned along the track centerline. Each sector alsohas a data field. The read element is aligned with the B servo burst anda write excitation field is applied to the write element. The amplitudesof an A burst and a C burst located in a following sector of the trackare then read to provide an A burst amplitude value and a C burstamplitude value. The steps of writing and reading are repeated for apredetermined number of times to obtain a set of A burst amplitudevalues and a set of C burst amplitude values. A value representing aninstability of the read element based on the set of A burst amplitudevalues and the set of B burst amplitude values is then calculated. Thehead is characterized based on the value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the variation of σ, the standarddeviation of the read center shift with respect to COV(TAA), thecovariance of the track average amplitude, as provided using a prior arttechnique.

FIG. 2A illustrates three servo bursts A, B and C that are written on atrack of a disk at a single frequency.

FIG. 2B is a graph illustrating the variation of the servo burstamplitude values V_(A), V_(B) and V_(C), of servo bursts A, B and C ofFIG. 2A respectively, with respect to the position of the read head of adisk drive.

FIG. 2C illustrates a single, burst amplitude profile representative ofthe three servo bursts A, B and C of FIG. 2B with respect to theposition of the read head of a disk drive.

FIG. 3 is a graph illustrating the magnetic read center shift for an MRhead, where each measurement is obtained after a write excitation.

FIG. 4 is a graph illustrating the normalized read center shift of aread head based on the correlation function provided in accordance withthe principles of the present invention.

FIG. 5 illustrates a hard disk drive which utilizes the method of thepresent invention.

FIG. 6 is a block diagram of portions of an integrated circuit readchannel in accordance with the present invention.

FIG. 7 illustrates the layout of a sector in a track in accordance withthe principles of the present invention.

FIG. 8 is a flowchart that illustrates one embodiment of correlationdetermination process provided in accordance with the principles of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of characterizing a read head.The technique of the present invention provides a correlation functionwhich accurately describes the instability of the read head based onmagnetic read center shifts and read head sensitivity changes.

A. PRINCIPLES OF OPERATION

I. Magnetic Read Center Shift

An expression for determining the MR read center shift more preciselywill now be derived and described. FIG. 2A illustrates three servobursts A, B and C that are written on a track of a disk at a singlefrequency. The servo bursts A, B and C are identical but offset inphase. In one embodiment, the B burst is centered along the centerlineof the track, while the A and C bursts have a common boundary at thetrack centerline. The B burst is located between the A and C bursts.FIG. 2B is a graph illustrating the variation of the servo burstamplitude values V_(A), V_(B) and V_(C) of servo bursts A, B and C ofFIG. 2A respectively, with respect to the position of the read head of adisk drive. Since A, B and C are identical but offset in phase, a singleoff track profile may be used to represent each V_(A), V_(B) and V_(C),as shown in FIG. 2C.

Co-pending U.S. application Ser. No. 08/819,295 entitled "Method andApparatus for Determining Write-Induced Magnetic Center Shifts in a ReadHead" filed Mar. 18, 1997, and assigned to the assignee of the presentinvention, provides a complete discussion on the determination ofmagnetic read center shifts in a read head. The subject matter of thisco-pending U.S. Application is incorporated herein by reference.

The expression for determining the MR read center shift is based onV_(A), V_(B) and V_(C), and utilizes V_(B) to compensate for the readsensitivity change.

Considering the off-track profile shown in FIG. 2C, let V_(Xn) be thereadback amplitude from the X burst at the nth reading. Thus,

    V.sub.An -V.sub.Cn =(K.sub.An -K.sub.Cn)Z.sub.n +d.sub.An -d.sub.Cn.(1)

where Z_(n) is the read center position for the head at the nth reading;K and d are the slope and the constant, respectively, for linearfittings describing the off-track profile at the nth reading.

For an initial state, n=0, the following expression is obtained:

    V.sub.A0 -V.sub.V0 =(K.sub.A0 -K.sub.C0)Z.sub.0 +d.sub.A0 -d.sub.C0(2)

Assuming that changes in the read width for the MR head are very smallunder the write excitations, then Z_(1n) -Z_(4n) and Z_(2n) -Z_(3n) areconstants. Thus, only K and d change as V_(B) changes. As a result, thefollowing expressions may be obtained:

    d.sub.An =d.sub.A0 V.sub.Bn /V.sub.B0                      (3)

    d.sub.Cn =d.sub.C0 V.sub.Bn /V.sub.B0                      (4)

    K.sub.An =K.sub.A0 V.sub.Bn /V.sub.B0                      (5)

    K.sub.Cn =K.sub.C0 V.sub.Bn /V.sub.B0                      (6)

Substituting Eqs. (3), (4), (5) and (6) into Eq. (1), the followingexpression is obtained.

    V.sub.An -V.sub.Cn =V.sub.Bn /V.sub.B0  (K.sub.A0 -K.sub.C0)Z.sub.n +(d.sub.A0 -d.sub.C0)!                                    (7)

Solving Z_(n) and Z₀ from Eqs. (2) and (7), the shift of the magneticread center from the initial reading n=0 to the mth reading n=m,δZ(0→m)=Z_(m) -Z₀, can be expressed as follows:

    δZ(0→m)= V.sub.B0 /V.sub.Bm (V.sub.Am -V.sub.Cm)-(V.sub.A0 -V.sub.C0)!/(K.sub.A0 -K.sub.C0)                          (8)

Eq. (8) provides the magnetic read center shift by compensating for theeffect of the read sensitivity change on the instability for MR heads.Since Eq. (8) only calculates the read center change, the initial readcenter location is not critical as long as V_(Bm) is the maximum valuefor the off-track profile at the mth reading. However, in most cases fora reasonably good MR head, the corresponding profile of the B burst nearthe center of the burst is fairly constant, so that if the read head isapproximately aligned with the center of the B burst, the value ofV_(Bm) near the center of the burst is approximately the maximum value.Therefore, one could still obtain reasonably accurate results even ifthe B burst written slightly off the center with respect to the commonboundary of the A and C bursts. As a result, the implementation of Eq.(8) is also simple.

FIG. 3 is a graph of the magnetic center shifts for an MR head based onmeasurements taken utilizing the technique provided in co-pending U.S.Application entitled "Method and Apparatus for determining Magnetic ReadCenter Shifts in a Read Head." In particular, the measurements are takenutilizing Eq. (8). As shown, the magnetic read center of the headchanged randomly and frequently under gated write excitation. Thenormalized read center shift for each MR head is defined by σ(readcenter shifts head)/(read width of the head) in terms of percentage.

Since the correlation between COV(TAA) and σ(shifts) is quite poor, thepresent invention provides a correlation function which accuratelydescribes the instability of a read head based on the magnetic readcenter shift.

II. A Correlation Analysis

If two variables have a linear relationship, the correlation coefficientbetween the two variables will be unity.

A. Cross-correlation coefficients of V_(An) and V_(Cn)

The change in the amplitude of the readback signals for unstable headsresults from two-physical phenomena: the read element sensitivitychanges and the magnetic read element center changes.

Two extreme cases for the instability will now be considered. First, theinstability of an MR head caused only by changes in the read sensitivityof the head (without considering shifting of the magnetic read center)will be considered. Thus, V_(An) and V_(Cn) will be increased ordecreased as V_(Bn) increases or decreases. As a result, the change inV_(An) and V_(Cn), δV_(An) and δV_(Cn) respectively, may be expressed asfollows:

    δV.sub.An =V.sub.An -V.sub.A0

    δV.sub.Cn =V.sub.Cn -V.sub.C0

For a symmetric off-track profile,

    K.sub.An =-K.sub.Cn

    δV.sub.An =δV.sub.Cn

For an asymmetric off-track profile:

    δV.sub.An αδV.sub.Cn

    V.sub.An αV.sub.Cn

Therefore, in this case, a cross-correlation coefficient of {V_(An) }and {V_(Cn) }, r(V_(A), V_(C)), equals to a positive unity, and can beexpressed as follows:

    r(V.sub.A,V.sub.C)=1                                       (9)

Thus, the instability of a head resulting purely from changes in theread sensitivity of the head provides a cross-correlation coefficient ofpositive unity.

Next, the instability of an MR head caused only by changes in themagnetic read center shifts (without considering any changes in readsensitivity) is considered. Thus, V_(Bn) =V_(B0) =const. V_(An)increases or decreases as V_(Cn) decreases or increases.

For the symmetric off-track profile:

    K.sub.An =-K.sub.Cn

    δV.sub.An =-δV.sub.Cn

For the asymmetric off-track profile:

    δV.sub.An α-δV.sub.Cn

    V.sub.An α-V.sub.Cn

Therefore, for this case, the cross correlation coefficient of {V_(An) }and {V_(Cn) }, r{V_(A),V_(C) }, equals to a negative unity, that is:

    r(V.sub.A,V.sub.C)=-1                                      (10)

Thus, the instability of a head resulting purely from changes in themagnetic read center shifts of the head provides a cross-correlationcoefficient of negative unity.

B. Correlation function describing magnetic read center shifts

Based on the above analysis, the write-induced magnetic read centershift for MR heads may then be characterized based on the combinedfactors of COV(TAA) (or COV(amplitude), i.e., COV(V_(A)) and COV(V_(C)))and the cross correlation of V_(A) and V_(C), i.e., r(V_(A), V_(C)). Byincluding both the cross correlation and COV(amplitude), the followingcorrelation function is obtained:

    M(V.sub.A, V.sub.C)= 1-r(V.sub.A, V.sub.C)!{ COV(V.sub.A)+COV(V.sub.C)!/2}.sup.2 /100       (12)

where ##EQU1##

N, i and j are integers. In one embodiment, i=j. In another embodiment,N=100.

If there is no shift in the magnetic read center, then r(V_(A),V_(C))=1, and the M(V_(A), V_(C)) function equals to zero. If theinstability results purely from the shift of the magnetic read center,then r(V_(A),V_(C))=-1, and the M(V_(A), V_(C)) function reaches amaximum value. In Eq. (12), COV(V_(A)) and COV(V_(C)) were utilized toscale M (V_(A), V_(C)).

B. APPLICATION

Referring to the drawings more particularly by reference numbers, FIG. 5illustrates a hard disk drive 10 which implements the apparatus andmethod of the present invention. The disk drive 10 includes a disk pack100 with a plurality of disks 102 that are collectively rotated by aspin motor 104. The spin motor 104 is mounted to a base plate 106. Alsomounted to the base plate 106 is an actuator arm assembly 108. Theactuator arm assembly 108 includes a number of read/write (R/W) heads110a-d mounted to corresponding flexure arms 112. The flexure arms 112are attached to an actuator arm 114 that can rotate about a bearingassembly 116. The assembly 108 also contains a voice coil motor 118which moves the heads 110a-d collectively relative to the disks 102.There is typically a single head for each disk surface. The spin motor104, voice coil 118 and the heads 110 are coupled to a number ofelectronic circuits 120 mounted to a printed circuit board 122. In thefollowing discussion, only one head 110 is referenced. The electroniccircuits 120 typically include a read channel circuit, amicroprocessor-based controller and a random access memory (RAM) device.

FIG. 6 is a block diagram of an electronic circuit 120 of the drive. Theelectronic circuit 120 includes a preamplifier 122 which is coupled to aread/write (R/W) channel circuit 124. The R/W channel circuit 124includes a R/W Automatic Gain Control (AGC), a filter circuit 126, afullwave rectifier 128 and a peak detector 130. The electronic circuit120 further comprises a microprocessor-based servo controller 132 whichincludes an analog-to-digital converter (ADC) 134, a digital signalprocessor (DSP) 136, a burst sequencer and timing circuit 138 and amemory 140, such as a random access memory (RAM) device.

The electronic circuit 120 is coupled to one of the magnetic heads 110which senses the magnetic field of a magnetic disk 102. When reading theservo information located in the servo field region 10 on the disk 102,the head 110 generates a read signal that corresponds to the magneticfield of the disk 102. The read signal is first amplified by thepreamplifier 122, and then provided to the R/W channel circuit 124. TheAGC data included in the read signal is provided to the R/W AGC andfilter circuit 126. The R/W AGC circuit in circuit 126 monitors the AGCdata provided by the read signal and the read signal is then filtered bythe filter circuit located in the R/W AGC and filter circuit 126. Thefullwave rectifier 128 rectifies the read signal and provides therectified read signal to the peak detector 130. The peak detector 130detects the amplitude of the read signal. The read signal is thenprovided to the ADC 134 which provides digitized samples of the analogread signal. The digitized signal is then provided to a digital signalprocessor (DSP) 136 which first reconstructs the burst amplitude valuescorresponding to the readings of the A, B and C bursts. Based on thereadings of the servo bursts A, B and C, the DSP 136 can determine thewrite-induced magnetic center shift for the read head.

FIG. 7 illustrates the layout of a sector in a track in accordance withthe principles of the present invention. Data is stored within sectorsof radially concentric tracks located across the disk 102. In accordancewith the principles of the present invention, a sector within a trackwill have an automatic gain control (AGC) field 150, a synchronization(sync) field 152, a gray code field 154 that identifies the track, anidentification (ID) field 156 that defines the sector, a servo field 158which includes a number of servo bursts A, B and C, a data field 160which contains the data and an error correction code field 162. Theservo bursts A, B and C may be written on a track of the disk 102 usinga spinstand, a head-media tester or a servo writer, as known in the art.In one preferred embodiment, the servo bursts A, B and C are identical,but offset in phase so that the A burst and the C burst have a commonboundary located at the track centerline and where the centerline of theB burst is coincident with the track centerline.

Once the servo bursts A, B and C have been written, testing of one ormore heads may begin. To determine the instability based on the readcenter shift in a R/W head 110, the R/W head 110 is first aligned withthe center of the B burst. A predetermined number of measurements ofV_(A) and V_(C) are then obtained, each of which is taken after a writeexcitation field has been applied to the R/W head 110. Based on themeasurements, r(V_(A), V_(C)), COV(V_(A)) and COV(V_(C)) are calculatedbased on Eq. (13), (14) and (15) respectively. Next, the correlationfunction M(V_(A), V_(C)) is calculated based on r(V_(A), V_(C)),COV(V_(A)) and COV(V_(C)), as provided in Eq. (12). The results are thenstored.

FIG. 8 is a flowchart that illustrates one embodiment of the correlationdetermination process in accordance with the principles of the presentinvention. Beginning from a start state, the process S100 proceeds toprocess step S102, where the R/W head 110 is approximately aligned withthe center of the B servo burst. The process S100 then proceeds toprocess step S104, where the counter N, for maintaining a count of themeasurements taken is set to 1. The process S100 then proceeds toprocess step S106, where a write excitation field is applied to the R/Whead 110. In particular, the R/W head 110 is directed to write in thedata field 160 of a first sector. The process S100 then proceeds toprocess step S108, where it directs the R/W head 110 to proceed to theservo field of the next sector to read the A and C burst amplitudevalues V_(AN), and V_(CN). In this initial case, N=1, so that V_(AN)=V_(A1) and V_(CN) =V_(C1). The values of V_(AN), and V_(CN) are thenstored, as shown in process step S110.

The process S100 then advances to decision step S112, where itdetermines if it has reached the predetermined number of measurements,NN. In one embodiment, NN is 100. It is apparent to one of ordinaryskill in the technology that NN may be greater or less than 100, and maybe selected according to accuracy requirements or according tospecification. If the predetermined number of measurements NN has notbeen reached, the process proceeds to returns to process step S106,where it applies a write excitation field to the R/W head 110. Theprocess S100 then proceeds to take another reading of the A and C burstamplitude values V_(AN), and V_(CN). If, at decision step S112, thepredetermined number of measurements NN has been reached, the processproceeds to process step S114, where it calculates r(V_(A), V_(C)),COV(V_(A)) and COV(V_(C)) based on the NN measurements taken. Next, theprocess S100 proceeds to process step S114, where it calculates M(V_(A),V_(C)) based on r(V_(A), V_(C)), COV(V_(A)) and COV(V_(C)). The processS100 then proceeds to process step S118, where it determines if M(V_(A),V_(C)) is larger than a predetermined threshold. In one embodiment, ifM(V_(A), V_(C)) is larger than the predetermined threshold, the R/W head110 is accepted as a qualified head. Otherwise, the R/W head 110 isrejected. The value of M(V_(A), V_(C)) will correspond to the standarddeviation of the magnetic center shift determined by the overall trackmisregistration budget. Accordingly, the predetermined threshold is alimit set by the standard deviation. The results are then stored, asshown in process step S120 and the process S100 terminates.

It should be noted that the process S100 may be implemented for MR headsor inductive heads to determine the write-induced magnetic center shiftsof the heads. In addition, the process S100 may be repeated for anynumber of heads. Moreover, the process S100 may be implemented at thedisk drive 100 level or at the servo writer level.

C. Experimental Results

To verify the effectiveness for the expressions in Eq. (8) and in Eq.(12), a group of MR heads with different coefficients of variances, COV,were tested. The results are shown in FIG. 4. Comparing with FIG. 1 withFIG. 4, it can be observed that the write-induced magnetic read centershift for MR heads may be more accurately characterized by using thetechnique of the present invention (through the application of Eq. (12))than by utilizing the coefficient of variance, COV. It should be notedthat the correlation function M(V_(A), V_(C)) is not a unique functionfor describing the shift of the magnetic center. A different functionadapting the above functions and techniques which involves COV(TAA) andthe cross correlation of V_(A) and V_(C) or δV_(A) and δV_(C) will alsocorrelate well with the shift of the magnetic read center.

It was found experimentally that δV_(An) α δV_(Cn) and δV_(Ak) α-δV_(Ck) almost always co-existed. This indicated that the instabilityindeed was combined effect of the changes in the read sensitivity andthe shifts in the magnetic read center. To minimize the effect of theinstability on track misregistration, the cross-correlation close topositive unity is always preferred. If the coefficient is lower than0.5, the contribution from the shift of the magnetic read center to theinstability could be significant. Where the cross correlationcoefficient is zero, the contribution to the instability from changes inread sensitivity and the shifts of the read center are equal. It shouldbe noted that the functions and techniques developed herein may beapplied not only to MR heads, but also to inductive heads. Thesefunctions and techniques could also be applied to servo systems tominimize track misregistration for both MR and inductive heads.

Through utilization of the apparatus and method of the presentinvention, a method and apparatus for determining instability andwrite-induced magnetic center shifts for a read head is provided. Inparticular, the write-induced instability of MR heads can be describedby combined effects of changes in read sensitivity and magnetic readcenter shifts for the MR heads. These two effects can be characterizedby a correlation analysis. Instabilities of the head resulting purelyfrom changes in the read sensitivity of the MR head bring about a unityof the coefficient. Instabilities of the head resulting purely fromshifts of the magnetic read center of the MR head bring about a negativeunity of the coefficient.

The present invention also provides a technique for determininginstability based on the magnetic read center shifts and readsensitivity changes in unstable heads. The present invention facilitatesreduction of track misregistration caused by write-induced magneticcenter shift, as well as the classification and qualification of readheads. In addition, the present invention also facilitates improvementof the read head design, thereby improving the read throughputperformance and consequently, increased product yields.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention not be limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those ordinarily skilled in the art.

What is claimed is:
 1. A method for characterizing a head in a harddrive assembly, based on a magnetic center shift and a read sensitivitychange in a read element on the head, said head also having a writeelement, comprising the steps of:(a) providing a disk attached to a harddisk drive assembly, said disk having a plurality of tracks each havinga track centerline, one of said tracks having a plurality of sectors,one of said sectors having a servo field with an A burst, a B burst anda C burst, where the A burst and the C burst have a common boundarylocated at a track centerline and where B burst has a center that ispositioned along the track centerline, each sector also having a datafield; (b) aligning the read element with the B servo burst; (c)applying a write excitation field to said write element; (d) reading anA burst and a C burst located in a following sector of said one of saidtracks to provide an A burst amplitude value and a C burst amplitudevalue; (e) repeating steps (c) and (d) for a predetermined number oftimes to obtain a set of A burst amplitude values and a set of C burstamplitude values; (f) calculating a value representing an instability ofsaid read element based on said set of A burst amplitude values and saidset of C burst amplitude values; and (g) characterizing said head basedon said value.
 2. The method as recited in claim 1, wherein step (f)comprises the steps of:(f.1) calculating cross-correlation of said setof A burst amplitude values and said set of C burst amplitude values;and (f.2) calculating a coefficient of variance of said set of A burstamplitude values; (f.3) calculating a coefficient of variance of saidset of C burst amplitude values; and (f.4) determining a valuerepresenting an instability of said read element based on saidcross-correlation, said coefficient of variance of said set of A burstamplitude values and said coefficient of variance of said set of C burstamplitude values.
 3. The method as recited in claim 2, furthercomprising the step of:(g) determining that the instability of said readelement is based solely on the read sensitivity change if thecross-correlation of said set of A burst amplitude values and said setof C burst amplitude values is positive unity.
 4. The method as recitedin claim 2, further comprising the step of:(g) determining that theinstability of said read element is based solely on the shift of theread center if the cross-correlation of said set of A burst amplitudevalues and said set of C burst amplitude values is negative unity. 5.The method as recited in claim 2, further comprising the step of:(g)determining that the instability of said read element is based solely onthe read sensitivity change if said value is zero.
 6. The method asrecited in claim 2, further comprising the step of:(g) determining thatthe instability of said read element is based solely on the shift of theread center if said value is a maximum value.
 7. The method as recitedin claim 1, wherein in step (d), said predetermined number of times is100.